Semiconductor fabrication involves complex processes for creating integrated devices within a semiconductor wafer and connecting the integrated devices so that they can communicate with each other. Generally speaking, semiconductor fabrication is divided into two phases of processing: front-end and back-end processing. Front-end processing relates to the process steps that form the integrated devices within the surface of semiconductor wafer substrates. Integrated devices may include active electrical devices such as transistors and passive electrical devices such as capacitors, inductors, and resistors. The front-end process steps include, but are not limited to, photolithography processes, etching processes, and doping or implanting processes. Back-end processing occurs after the completion of the front-end processing steps. The back-end processing steps form multiple and stacked “metal layers” on top of a semiconductor wafer. Each metal layer includes electrically conductive traces for connecting various integrated devices within the semiconductor wafer. A dielectric layer formed of, for example, an oxide separates each of the metal layers and also provides for connectivity between each metal layer through conductive vias. Salicidation processes connect the conductive traces of the metal layers to the integrated devices within the semiconductor substrate. Salicidation may use a material such as a silicide-metal alloy (e.g., nickel or cobalt).
Current semiconductor devices increasingly use resistors to control the timing of transmitted signals within various integrated devices and electrical components. These resistors are formed in the semiconductor wafer using front-end processing steps. Such resistors may be poly-silicon and/or island resistors. Polysilicon resistors are formed on top of a semiconductor wafer substrate and gain a certain resistance through doping or implanting processes. For example, polysilicon may be doped with a material such as boron or arsenic. On the other hand, island resistors are formed within a semiconductor wafer substrate.
Unfortunately, real estate in a semiconductor wafer substrate is usually very limited. The need to create passive components for e.g., resistors and capacitors on Si substrates makes it very challenging to squeeze more transistors onto a wafer substrate. In view of the foregoing, there are continuing efforts to provide improved techniques for fabricating electrical devices, such as resistors, into semiconductor circuits.